fix: Isolate & Workaround the Stack_overflow raised by 'Experiment'

* Add some (commented-out) debug code facilities;
* Add some TODO/FIXME/XXX remarks;
* Workaround the issue by ignoring the 'exercise.descr' string in editor_lib.ml
  (otherwise its encoding-then-*decoding* fails)

Author: erikmd

src/app/learnocaml_exercise_main.ml

@@ -90,18 +90,24 @@ let () =
 
   (* if we came from a true exercise we search in the server.
    In the other case we get the exercise information from the Local storage *)
+  (* FIXME: for debug purposes; to be removed:
+  let ok str = lwt_alert ~title:"TEST1" ~buttons:["OK", (fun () -> Lwt.return_unit)]
+                [ H.p [H.txt (String.trim str)] ] in *)
   let exercise_fetch = match idEditor id with
     | false -> token >>= fun token ->
                retrieve (Learnocaml_api.Exercise (token, id))
 
     | true -> let proper_id = String.sub id 1 ((String.length id)-1) in
-              Lwt.return ((Editor_lib.get_editor_state proper_id).Editor.metadata,
-                          (Editor_lib.exo_creator proper_id ),
-                          None)
+              let state = Editor_lib.get_editor_state proper_id in
+              (* FIXME: for debug purposes; to be removed:
+              ok state.Editor.metadata.title >>= fun () -> *)
+              let exo = Editor_lib.exo_creator proper_id in
+              Lwt.return (state.Editor.metadata, exo, None)
   in
 
   let after_init top =
     exercise_fetch >>= fun (_meta, exo, _deadline) ->
+
     begin match Learnocaml_exercise.(decipher File.prelude exo) with
       | "" -> Lwt.return true
       | prelude ->

src/editor/editor_lib.ml

@@ -293,18 +293,21 @@ let genTemplate top ?(on_err = fun () -> ()) sol =
     else Lwt.return (let () = on_err () in "")
 
 (* ---- create an exo ------------------------------------------------------- *)
+
 let exo_creator proper_id =
   let exercise = (get_editor_state proper_id).exercise in
   let read_field field =
       match field with
-      | "id"-> Some exercise.id
+      | "id" -> Some exercise.id (* XXX = proper_id *)
       | "prelude.ml" -> Some exercise.prelude
       | "template.ml" -> Some exercise.template
-      | "descr.md" -> Some exercise.descr
+      | "descr.md" -> (* FIXME: SHOULD BE 'Some exercise.descr' *)
+         Some "<p>Note: question export disabled for the time being; otherwise descrs_from_string raises a Stack_overflow on strings of size 14000.</p>"
       | "prepare.ml" -> Some exercise.prepare
-      | "test.ml" -> Some exercise.test
       | "solution.ml" -> Some exercise.solution
-      |  "max_score" -> Some (string_of_int exercise.max_score)
+      | "test.ml" -> Some exercise.test
+      | "max_score.txt" -> Some (string_of_int exercise.max_score)
+      | "depend.txt" -> None (* TODO: Add support *)
       | _ -> None
   in
   Learnocaml_exercise.read

src/repo/learnocaml_exercise.ml

@@ -21,6 +21,8 @@ type t =
     dependencies : string list;
   }
 
+(* XXX: a change to this type may require another change in editor_lib.ml *)
+
 let encoding =
   let open Json_encoding in
   conv
@@ -192,7 +194,7 @@ module File = struct
 
   let depend =
     { key = "depend.txt" ; ciphered = false ;
-      decode = (fun v -> Some v) ; 
+      decode = (fun v -> Some v) ; (* XXX should this produce None sometimes? *)
       encode = (function 
                 | None -> "" (* no `depend` ~ empty `depend` *)
                 | Some txt -> txt) ;
@@ -289,7 +291,7 @@ module File = struct
        *         return ()
        *       with _ -> return ()
        * in *)
-      let descrs = ref [] in
+      let descrs : (string option * string) list ref = ref [] in
       let rec read_descr lang = function
         | [] ->
            (* If there are no extensions to try, we just give up. *)
@@ -314,23 +316,32 @@ module File = struct
            | Some raw ->
               (* If it does, we apply the function, add the
                  description to [!descrs] and return. *)
-              descrs := (lang, f raw) :: !descrs;
+              descrs := (lang, f raw) :: !descrs ;
               return ()
       in
       let markdown_to_html md =
         Omd.(md |> of_string |> to_html)
       in
       let read_descrs () =
         let langs = [] in
+        (* XXX: Really [] ? *)
         let exts = [
             (Filename.extension descr.key, fun h -> h) ;
             (".md", markdown_to_html)
           ] in
         join (read_descr None exts :: List.map (fun l -> read_descr (Some l) exts) langs)
         >>= fun () ->
-        ex := set descr
-                (List.map (function (None, v) -> "", v | (Some l, v) -> l, v) !descrs)
-                !ex;
+        let res = (List.map (function (None, v) -> "", v | (Some l, v) -> l, v) !descrs) in
+        (* let res' = descr.encode res in
+        let stack_overflow = descr.decode res' in () *)
+
+        (* Also reproducible with:
+        let html = (* markdown_to_html md (* length = 13711 *) *)
+          String.init 14000 (fun n -> if n mod 2 = 0 then 'A' else ' ') in
+        let res' = descrs_to_string [("", html)] (* length = 13857 *) in
+        let stack_overflow = descrs_from_string res' in () *)
+
+        ex := set descr res !ex;
         return ()
       in
       join